The present invention relates generally to optical systems. More particularly, the invention includes a method and structure for increasing output power in fiber laser systems. Merely by way of example, the invention has been applied to a temperature controlled length of optical fiber characterized by reduced non-linear optical effects. But it would be recognized that the invention has a much broader range of applicability.
Fiber lasers have advanced to become robust and efficient high powered infrared laser sources. Average output powers of tens of kilowatts are currently available in commercial fiber laser systems. FIG. 1 is a schematic of a conventional fiber laser system in a master oscillator/fiber amplifier configuration. The seed laser 120 emits a low power optical signal that is coupled into the amplifier section 140 through an optical isolator 130. The optical isolator protects the seed laser from light counter-propagating back through the amplifier section. The amplifier section consists of a length of gain fiber that is pumped by one or more pump lasers 160 (typically diode lasers) through a pump coupler 150. The gain fiber may be multi or single spatial mode(s), polarization random or maintaining, cladding pumped or core pumped, and with a variety of atomic dopants, depending on the emission and pumping wavelengths. The pump laser light is absorbed by the dopants in the gain fiber, raising the dopants into an excited state. The seed is amplified through stimulated emission as it interacts with the excited dopants.
A laser may be constructed using an optical fiber as the gain medium and are pumped with optical energy. Fibers are typically glass-type materials, though may be crystalline or glass-nano-crystal composites, and are commonly doped with atoms from a set {Yb, Er, Nd, Pr, Tm, Cr}. The fiber laser gain architectures include single pass gain, confined cavity, master oscillator/fiber amplifier(s), and the like. Many variants of fiber laser design are commonly employed, e.g. multiple gain stages with multiple pumps, inclusion of various filtering elements, a delivery fiber at the output of the laser, and use of forward- and/or backward-propagating pumps. Fiber lasers can operate with a wide range of output parameters to satisfy the varying constraints of an application, with the specifications of the individual fiber amplifier subsystems driving the down-selection of components. The output emission of a fiber laser can be specified with the average output optical power, peak output optical power, temporal pulse width, center optical wavelength, polarization, spatial mode, and spectral bandwidth.
The components selected to achieve these characteristics include a master oscillator that generates linear polarized infrared laser radiation, a polarization-maintaining optical isolator that attenuates the backward propagating light from the fiber amplifier while transmitting the forward propagating light from the master oscillator, and a fiber amplifier that contains a power amplifier that amplifies the master oscillator emission transmitted through the isolator. Examples of fiber amplifiers include a polarization-maintaining, large-mode-area, double-clad {Yb—, Er—, Yb:Er—} doped gain fiber. Components also include one or more diode pump lasers typically based upon AlGaAs/GaAs designs that emit light of selected wavelengths from a range 910-985 nm, and a means of coupling the emission from the diode pump lasers into the gain fiber.
Despite the progress made in the development of fiber laser systems, there is a need in the art for improved methods and systems related to fiber laser amplifiers and fiber laser systems.